IN FLOOR PEX HEATING PERFORMANCE Q
We put in hydronic heat in a 500 sq ft shop. Two loops of 250 ft 1/2 pex/ 5 1/2 inches of concrete over 2 inch starfoam / the pex is held in the center by two layers of 1/2 rebar 2 ft on center offset to appear as 1 ft on center from the top/ we got the plumbing mechanical from radiantec/ the heat source is 4500wt el water heater (more available heat than necessary). It is a closed system.
Here’s my question: The heat appears to be moving by convection. It hasn’t become very cold yet (about 30F low for the day). The thermostat is set at 64. The in/out is reading about 74F/74F. In the picture the check valve is right above the pump. This is a one zone setup.
Does it seem reasonable that the hot water would move by convection through this – and if the 74 was too warm how would you regulate it? By turning back the heat on the water heater?
Replies
Couple of things. Is the pump hot? If it is, it is probably air locked.
If the pump is running, try closing the valves on the return to slow down the flow. Infloor usually needs about 1/2 gpm per loop. That pump moves a lot more than that. Slowing the flow allows the concrete to "grab" the heat.
"is the pump hot"? No, the pipes nor the pump feel hot (74F). If you kick the thermostat up above the 74 the pump runs fine and it circulates. We haven't got into cold enough weather yet to challenge the thermostat set at 64.We mounted the pump in that position as recommended in the plumbing mechanical instructions. They said to not have it pumping down (it would tend to lose prime that way). I forgot to mention we filled the system using a home made vacuum system used in automotive air conditioning. That worked great. We filled it with the special chry-tek non toxic antifreeze for hydronic heat from Manard's.
Looking at your picture, there is 2-3 feet or elevation to the top of the loop after the WH. This will allow the heated water to rise, and if it gives up some heat in the process, it will then "fall" into the down leg to the floor. In turn the now much cooler water in the floor is being pushed up into the WH. The process sets up a natural circulation that can continue indefinitely.
There are spring loaded check valves you can add to the loop which only allow water movement when pressure (pump on) is applied. Otherwise the spring loaded valve halts all movement. They are relatively cheap and would likely solve your convection issue.
That sounds reasonable to me. We have noticed a little pressure showing up on by the ballast if the water heater kicks on - about 15 psi - then it will drop as I suppose the water moves. It isn't a problem yet, the shop is running and the owner only had one day when indian summer hit and he wondered if he was going to have to open the door. I told him he could always shut the water heater off. Boy that's really a nice heat. The last time I was there I could feel my feet getting warm. We insulated it good. R Board on inside of 9 inch walls total R35 and ceiling R 45. I'm glad the consultant had us go to 2 inch starfoam under and perimeter. We duct taped our rebar together and put it on starfoam chairs to eliminate sharp edges around the pex.We came up with a little trick that worked great cutting the starfoam with the hot wire. I struck a chalk line and stuck nails on the line to guide the hot wire.
Ok, now I understand. (Us plumbers can be a little slow.)
If you are getting heat migrating, then insulating the pipes "might" help. Does the pump model end with FC? If it does then you already have a flow check built in like JO suggested.
Easiest solution is turn off the water heater untill it is needed. Little more expensive solution is to put in a mag starter so the stat can turn the water heater on/off.
BTW, I like the chairs.
There is a check valve the exit (top) of the pump.The starfoam chairs - I was amazed how easy it was to hold the 6 x 6 pieces down against a board and run a spade bit through to make the channel for the re-bar. I mean it amazed me how the bit tracked itself, naturally hugging the edge.Another thing that amazes me - over 35,000 lbs of concrete resting on 2 inches of starfoam. Thanks for mentioning that other option - the mag start for the water heater - hadn't thought of that.
35,000 = about 1/2 psi.